There has been hitherto known an optical scanning-type image forming apparatus which scans illumination light from the tip part of an optical fiber toward an object and detects detection light reflected and scattered by the object or fluorescence generated in the object. In such apparatus, the optical fiber is held in part with the tip part for emitting the illumination light being oscillatable, and an actuator is disposed in the vicinity of the supporting part, so as to vibrate the optical fiber, to thereby scan the detection light on the object.
Optical fibers are not always constant in its properties (such as Young's modulus and density), and susceptible to aging due to ambient environmental change such as temperature change, aging of the constituent members, and impact between an object during use. Members such as actuators including piezoelectric elements and adhesives forming the drive mechanism also vary in property with time. Such aging of properties of the optical fiber and the drive mechanism affects resonance frequency at the leading end of the optical fiber, attenuation coefficient (Q value) of the vibration, and the driving force of the drive mechanism, with the result that the scanning pattern of the optical fiber will deviate from the scanning pattern originally intended, as will be explained in detail with reference to FIGS. 1 and 2.
FIG. 1 illustrate, as a simplified example, the scanning of optical fibers along a circular pattern, in which: FIG. 1A shows a pattern of an optical fiber tip in the X-direction; FIG. 1B shows a pattern of an optical fiber tip in the Y-direction; and FIG. 1C shows an optical scanning pattern in the XY plane. The vibration of the fiber tip in the X-direction is different in phase by 90 degrees from the vibration of the fiber tip in the Y-direction, and thus the fiber tip renders a circular pattern. Meanwhile, FIG. 2 illustrate the scanning of the optical fiber after deterioration with age. Variations in resonance frequency, Q value of the vibration, and driving force of the drive mechanism affect the phase and amplitude, as can be seen in the pattern of the fiber tip in the X-direction and Y-direction of FIGS. 2A and 2B. As a result, the pattern of optical scanning on the object is also deformed as illustrated in FIG. 2C.
Without being limited to the aforementioned case of circular pattern, a scanning pattern 101 in a spiral scan, for example, is also deformed as illustrated by the solid line of FIG. 3, from a pattern (broken line) 102 originally intended. In a case where the pattern of optical scanning has been changed as described above, the resulting image of the object will suffer distortion to be different from the actual object when the image has been generated by mapping pixel data on a two-dimensional coordinates based on the pattern of the optical fiber originally anticipated. More specifically, as illustrated in FIG. 4, the actual pattern (plotted by white circles at certain time intervals) will be deviated in amplitude and phase from an ideal pattern (plotted by black circles at certain time intervals). In particular, when a phase difference θn is generated, the original image shown in FIG. 5A will be formed as an image circumferentially distorted in the center as shown in FIG. 5B.
To overcome such problem, JP2008514342A (Patent Literature (PTL) 1) illustrates the use of a scanning position detecting means such as position sensor devices (PSD), so as to obtain coordinate values of the actual scanning pattern and create a lookup table having information on the coordinate values, to thereby correct the coordinate to be applied to the pixels based on the lookup table.
Further, US20080165360 (PTL 2) suggests, as a method of correcting image distortion using a reflection image, storing a specific reference chart in a memory or the like and comparing the reference chart with a scanning pattern actually obtained.